This research is designed to clarify the role of hormonal receptors in thermogenic tissue in the increased thermogenic response to norepinephrine observed in winter acclimatized subarctic small mammals and to identify hormones which regulate receptors in thermogenic tissue of cold exposed and seasonally acclimatized subarctic small mammals. The number and binding properties of beta-adrenergic receptors in brown fat of small subarctic mammals (voles) will be studied by measuring the specific binding of the labeled beta-adrenergic antagonist (-) (3H) dihydroalprenolol to the receptors in brown fat membrane preparations. The number and affinity of the receptors and the kinetics and specificity of binding will be assessed in brown fat of voles collected from their natural habitat during all seasons of the year. The effects of cold and hormones on the quantity and quality of beta-adrenergic receptors in brown fat of subarctic voles will be evaluated after exposure to prolonged cold and chronic treatment with norepinephrine, thyroid hormone, thyroid blocking agent or melatonin. The proposed research should contribute to an understanding of basic mechanisms involved in responsiveness of tissues to hormones and should further our knowledge of the mechanisms by which one hormone regulates the end organ receptors of another hormone. These findings should be of general significance in understanding the physiological and pathological regulation of biological processes.